It is known that the surfaces of glass articles can be strengthened by producing a compressive stress layer at the surface of the article. Chemical tempering has been one common method of achieving such strengthening. Such method creates a positive compressive stress at the surface primarily by a chemical alteration of the surface area of the article. One method involves the exchange of ions at the glass surface by ion exchange reactions such as by the exchange of larger ions, i.e., potassium ions, for smaller sodium ions in the glass. The difficulties and expense of such treatments have prevented their widespread adoption in treating glass articles, except for special purposes.
The prior art has also coated glass bottles, especially those which contain pressurized liquid contents under normal conditions of use. The problems associated with glass breakage have been given extensive attention by researchers in the art with varied results. A common approach to improve glass container strength has been to coat or cover the glass container essentially overall to prevent or minimize its damage in normal conditions of use.
A widely-used commercial coating is a foamed plastic shield or tubular sleeve applied to the sides and a portion of the neck, heel and bearing surfaces of the bottle, frequently leaving the remainder of the bottom uncovered. Such coating appears to exhibit beneficial effects principally as a shock absorber during shipping and handling. Many consumers find such covered bottles to be esthetically appealing where the sleeve or label is capable of being decorated with patterns representing the product normal trade dress and trademarks. While the covering is a plastic film or foam, and thereby relatively soft, it is not easily marred or damaged due to its resiliency and thickness which is normally of the order of about 5 to 20 mils.
Further, many food products are currently being cooked in microwave ovens in the home or in vending areas, frequently in the shipping and delivery container. Presently, food packaged in metal cans must be transferred to other vessels for cooking in microwave ovens. Since foods can be microwave cooked while in glass containers, the need for additional disposable dinnerware or washing of reusable dinnerware is obviated. However, glass containers made of soda-lime glass do not consistently have adequate thermal shock strengths to withstand microwave heating and cooling by the user. The subject coating which is applied only on the bottom or lower portion of the container provides significant increases in thermal shock values.
The use of a thin frangible polymeric coating on the lower part of a jar or bottle not only increases thermal shock strengths, but also permits detection of glass cracks because the coating will also crack due to its brittle nature. Currently used "functional coatings" which are usually soft and distensible are not brittle and may not fracture with the glass. Actually many such coatings are flexible and employed to retain the glass on breakage. In such cases, the food product can leak through the glass crack and be contained by the flexible coating. Such condition could lead to spoilage of the food due to contact with bacteria on the coating. If this condition is not discovered, the spoiled food could be consumed subjecting the consumer to a health hazard.